CANE SUGAR. 



Failing these appliances, the prevention of scale can only be controlled by 

 care in clarification. Excessive scale in evaporators can often be traced to bad 

 clarification and imperfect settling, that is to say, to lack of craft skill on the 

 part of the responsible workman ; too little lime often results in a badly 

 settling juice. 



Yarious mechanical devices have been employed to diminish the deposit of 

 scale. Of these may be mentioned that of Novak, 3 who suspends from the 

 dome of the vessel chains which hang down in the tubes ; the chains either 

 depend from springs or are supported in pairs from the end of a suspended 

 pivoted arm ; the motion of the liquid keeps the chains moving against the 

 sides of the tubes, and a part of the scale also deposits on the chains 

 themselves. 



Lagrell and Chantrelle 3 devised the scheme of placing a hollow rod, of 

 nearly the same specific gravity as that of the liquid in each vessel, inside 

 each tube ; along this rod is cut a spiral ; the motion of the liquid keeps this 

 rod continually rotating against the side of the tube. 



The Lillie apparatus is designed so that the direction of flow can be 

 changed at will, and it is claimed that this change of movement prevents or 

 lessens the deposit of scale. 



The chief bodies that occur in the deposit of scale are the silicates, phos- 

 phates, and sulphate of lime ; the two former seem to be of general occurrence ; 

 but a sulphate scale in factories not employing sulphur does not appear to be 

 universal ; it does not occur except in small quantities in the analyses quoted 

 below due to Pellet and Geerligs, but occurs in large quantities in some of 

 those due to Peck 13 of Hawaiian scales. 



Phosphate of lime does not, according to Geerligs and Tervooren 14 , occur 

 in juices in solution, but in a colloid state, and hence does not occur in scales 

 from juices which have been filtered, and they state that the same is also true 

 of silica. Peck 13 , however, has found phosphate of lime in a state of true 

 solution, the amount decreasing with the quantity of lime used in clarification. 



Prom the analyses quoted below it will be seen that it is in the earlier 

 vessels that phosphates are deposited, silicates predominating in the later ones. 



The ' fats ' that occur in scales are due to the cane wax and also, 

 according to Shorey 16 , to the decomposition of the lecithins of the cane. 



The removal of the scale is only to be obtained by mechanical means with 

 scrapers and wire brushes, preceded by a preliminary treatment with appro- 

 priate solvents. Phosphate scales are best treated with hydrochloric acid, and 

 silicate scales with caustic soda, in 1 per cent, solution. Sulphate scales are 

 more troublesome to remove, and are best treated by first boiling with sodium 

 carbonate and then with hydrochloric acid ; the action of the sodium carbonate 

 is to convert the sulphate of lime to carbonate, which is then attacked by the 

 acid. 



336 



